The present invention relates to a method and apparatus for rejuvenating polishing slurry to be recovered and recycled in a chemical-mechanical polishing (CMP) process of a substrate, typically a semiconductor wafer.
Recently, in a process of fabricating transistors on a semiconductor wafer, a CMP process is often carried out to increase the uniformity of the wafer surface by planarizing an interlevel dielectric film thereon, for example. In the CMP process, polishing slurry, in which fumed or colloidal silica particles are dispersed as abrasive grains in an alkaline aqueous solution, for instance, is used.
The size of primary colloidal silica particles is between 20 and 30 nm. When these primary particles agglomerate together, secondary colloidal silica particles with a size of about 200 nm are formed. On the other hand, the size of fumed silica particles, which are obtained at the beginning of preparation by finely pulverizing their flakes greater in sizes, is about 200 nm.
The polishing slurry contains various contaminants such as abrasive grains with their properties degraded during polishing, pad debris that has been removed from a pad through conditioning and abraded parts of the workpiece (e.g., wafer). During polishing or waste recovery, a great number of those secondary particles might agglomerate together due to a significant change in hydrogen ion exponent (pH) or externally applied kinetic energy. As a result, excessively large particles with a size of 700 to 1500 nm might also exist in the slurry. If such chemically deteriorated, consumed slurry is used again as it is, then the uniformity of the wafer surface may be adversely affected or micro-scratches may be formed thereon. Thus, once used for polishing, the slurry recovered is usually recovered and discarded.
However, since the CMP process using slurry is carried out at an increasing number of facilities these days, increase in costs required for safe waste disposal and environmental protection is coming closer to an alarming level. To cope with such a problem, various techniques have been suggested to recycle the once-used slurry by recovering and rejuvenating it.
FIG. 21 schematically illustrates an arrangement of a prior art polishing slurry recovery system as disclosed in Japanese Laid-Open Publication No. 8-115892, for example.
In this polishing slurry recovery system, first, used polishing slurry in a slurry tank 501 is introduced into a microfiltration unit 502, thereby filtering out various contaminants in excessively large sizes of more than 500 nm and agglomerated abrasive grains from the slurry. The slurry, which contains remaining particles that have not been filtered out by a filter of the microfiltration unit 502, is returned into the tank 501 and then the slurry in the tank 501 is passed through the microfiltration unit 502 again. After the slurry has been circulated several times in this manner, the slurry with increased concentrations of large contaminants and agglomerated particles is drained as waste through a waste line.
The other part of the slurry, which has been passed through the filter of the microfiltration unit 502, is passed through a processed slurry tank 503 and then introduced into a ultrafiltration unit 504, thereby filtering out fine contaminants and fine abrasive grains in sizes of less than several tens nm from the slurry. In this case, the slurry, which contains particles that have not been filtered out by a filter of the ultrafiltration unit 504, is circulated by being returned into the processed slurry tank 503. After the slurry has been circulated several times in this manner, slurry, containing abrasive grains in sizes between several tens nm and 500 nm, is recovered with those fine contaminants and fine abrasive grains filtered out.
In the prior art polishing slurry recovery system, a considerable amount of solids, which contains large particles such as the abrasive grains and secondary particles thereof, is filtered out by the microfiltration unit 502. Thus, it is important to select an appropriate combination of pore diameters for the filters of the micro- and ultrafiltration units 502 and 504. For example, if the pore diameter of the filter for the microfiltration unit 502 was increased to avoid filter clogging, then those large particles and contaminants, which must have been filtered out otherwise, could not be filtered out, thus creating micro-scratches. Nevertheless, if the pore diameter was decreased, then as much as several tens percent of the solids containing the primary and secondary particles of the abrasive grains is captured unintentionally. As a result, those filters are clogged up and the recovery and supply of the polishing slurry come to a halt.
An exemplary countermeasure is disclosed in Japanese Laid-Open Publication No. 10-118899. In accordance with this prior art technique, a winding filter with a pore diameter of 25 to 100 xcexcm, which is larger than the size of large particles, is used, thereby avoiding rapid clogging due to crosslinking of contaminants such as gels and yet filtering out pad debris, contaminants and large particles. According to this technique, filter clogging can be suppressed to a certain degree. However, abrasive grains, which constitute part of the solids of the large particles, are also lost at the same time.
Accordingly, every time the polishing slurry, drained from a CMP polisher, is rejuvenated in accordance with the prior art polishing slurry recovery technique, the abrasive grains in the slurry are partially lost unintentionally, thus decreasing the recovery rate of the abrasive grains.
An object of the present invention to rejuvenate used polishing slurry easily and substantially without losing abrasive grains, decreasing the polishing rate or creating micro-scratches so that the recycled polishing slurry contains abrasive grains with rejuvenated capabilities almost comparable to those of non-used, fresh polishing slurry.
To achieve this object, in the inventive method and apparatus for rejuvenating polishing slurry, large particles, which have been made up of abrasive grains agglomerated together, are re-dispersed by applying electromagnetic field or ultrasonic radiation or adding a dispersant thereto.
Specifically, an inventive method for rejuvenating a polishing slurry that has been used for a chemical-mechanical polishing process includes the steps of: a) recovering the polishing slurry; and b) re-dispersing abrasive grains contained in the polishing slurry recovered.
According to the inventive polishing slurry rejuvenating method, even if large particles have been made up of abrasive grains agglomerated together while the polishing slurry, which has been once used for chemical-mechanical polishing, is being recovered and rejuvenated, those large particles can be re-dispersed. Thus, particles in sizes suitable for the polishing process can be obtained with almost no abrasive grains lost from the polishing slurry. In this manner, the used polishing slurry can be rejuvenated easily so that the polishing slurry recovered contains abrasive grains with rejuvenated capabilities comparable to those of fresh polishing slurry.
In one embodiment of the present invention, the step b) may include the step of adding a dispersant to the polishing slurry recovered.
In this particular embodiment, the dispersant preferably includes an anionic high-molecular surfactant.
In another embodiment of the present invention, the step b) may include the step of applying an electromagnetic field to the polishing slurry recovered.
In an alternative embodiment, the step b) may include the step of applying ultrasonic radiation to the polishing slurry recovered.
In the latter embodiment, the ultrasonic radiation is preferably applied at an output power of about 400 to about 800 W and at a frequency of about 10 to about 30 kHz.
As another alternative, the method may further include the steps of: c) filtering out fine particles in sizes equal to or smaller than a lower allowable limit from the polishing slurry recovered; d) filtering out large particles in sizes equal to or greater than an upper allowable limit from the polishing slurry recovered; and e) controlling a hydrogen ion exponent (pH) of the polishing slurry recovered. All of these steps c), d) and e) are performed after the step a) has been carried out. In such an embodiment, the fine particles in various sizes equal to or smaller than the lower allowable limit and the large particles in various sizes equal to or greater than the upper allowable limit are both removed from the polishing slurry recovered. Thus, the polishing rate does not decrease or the micro-scratches are not formed, either. Furthermore, since the pH of the polishing slurry recovered is controlled, the abrasive grains contained in the polishing slurry recovered are less likely to agglomerate together during the step of re-dispersing. As a result, decrease in amount of particles is avoidable.
In this particular embodiment, the step b) is preferably performed before the step c) is carried out. Then, it is possible to prevent a microfiltration film from being clogged up while the fine particles are being filtered out from the polishing slurry recovered.
An inventive apparatus for rejuvenating a polishing slurry that has been used for a chemical-mechanical polishing process includes: means for recovering the polishing slurry; and means for re-dispersing abrasive grains contained in the polishing slurry recovered.
In one embodiment of the present invention, the re-dispersing means may re-disperse the abrasive grains by adding a dispersant to the polishing slurry recovered.
In this particular embodiment, the dispersant preferably includes an anionic high-molecular surfactant.
In another embodiment of the present invention, the re-dispersing means may re-disperse the abrasive grains by applying an electromagnetic field to the polishing slurry recovered.
In an alternative embodiment, the re-dispersing means may re-disperse the abrasive grains by applying ultrasonic radiation to the polishing slurry recovered.
In the latter embodiment, the ultrasonic radiation is preferably applied at an output power of about 400 to about 800 W and at a frequency of about 10 to about 30 kHz.
As another alternative, the apparatus may further include: means for filtering out fine particles in sizes equal to or smaller than a lower allowable limit from the polishing slurry recovered; means for filtering out large particles in sizes equal to or greater than an upper allowable limit from the polishing slurry recovered; and means for controlling a hydrogen ion exponent (pH) of the polishing slurry recovered.
In this particular embodiment, the large particle filtering means preferably includes a filter with a pore diameter between 100 xcexcm and 200 xcexcm, both inclusive.